Hydrophobic membrane Materials for filter venting applications

ABSTRACT

The present invention relates to filtration media having both hydrophobic (water-repellent) properties. The filtration media are produced using a fluorothermoplastic material, such as a terpolymer of tetrafluoroethylene, vinylidene fluoride, and hexafluoropropylene. The invention also relates to methods of preparing such filtration media using casting solution phase inversion.

FIELD OF THE INVENTION

[0001] The present invention relates to filtration media having bothhydrophobic (water-repellent) properties. The filtration media areproduced using a fluorothermoplastic material, such as a terpolymer oftetrafluoroethylene, vinylidene fluoride, and hexafluoropropylene. Theinvention also relates to methods of preparing such filtration mediausing casting solution phase inversion.

BACKGROUND OF THE INVENTION

[0002] Hydrophobic filters are used in filtration of gases, in ventingfilters, and as gas vents. These hydrophobic filters allow gases andvapors to pass through the filter while liquid water is repelled by thefilter.

[0003] Polytetrafluoroethylene (PTFE) has been the most commonly usedmaterial in filters for gas venting. PTFE is chemically and biologicallyinert, has high stability, and is hydrophobic. PTFE filters thereforeallow gases to be selectively vented while being impervious to liquidwater.

[0004] Hydrophobic membranes are used as filters in healthcare andrelated industries, for example, as vent filters for intravenous (IV)fluids and other medical devices. In the healthcare industry, themembrane is sterilized before use. PTFE membranes can be sterilized forthese health-related applications with steam or by chemicalsterilization without losing integrity.

[0005] Treating PTFE membranes with steam can cause pore blockage due tocondensation of oil from the machinery used to generate the steam. Theresulting loss of air permeability reduces the membrane's ability toserve as an air vent. Although chemical sterilization minimizes exposureof the membrane to oil, chemical sterilization uses toxic chemicals andcan generate byproducts, which cause waste disposal problems. Ionizingradiation has also been used for sterilization of materials used inmedical and biological devices. PTFE may become unstable when exposed toionizing radiation. Irradiated PTFE membranes have greatly reducedmechanical strength and cannot be used in applications where they aresubjected to even moderate pressures.

[0006] Perhaps the two biggest drawbacks to PTFE as a filter for ventinggases are the high cost and the low air permeability of PTFE membranes.PTFE membranes are formed by extruding and stretching PTFE. Processingmethods to form PTFE membranes may be expensive. Furthermore, theextruding and stretching processes used to form PTFE membranes create amembrane which has relatively, low air permeability.

[0007] As a result, efforts have been made to identify alternativesubstrates which are less expensive and have higher air permeabilitythan PTFE and which can be modified to be hydrophobic.

[0008] Coating filtration substrates allows one to retain the desirablebulk properties of the substrate while only altering the surface andinterfacial properties of the substrate. Coating substrates to increasetheir hydrophobic properties has not been very practical, because thecoatings can reduce permeability. Furthermore, many of the coatingmaterials are expensive.

[0009] Scarmoutzos (U.S. Pat. No. 5,217,802) modified the surface ofsubstrates made of nylon, polyvinylidene difluoride (PVDF), andcellulose by treating the substrate with a fluorinated acrylate monomer.The substrate was sandwiched between two sheets of polyethylene, and themonomer was polymerized by exposing to ultraviolet light. The resultingcomposite filters had hydrophobic and oleophobic surfaces. The airpermeability of the filters decreases with time.

[0010] Moya (U.S. Pat. No. 5,554,414) formed composite filters frompolyethersulfone and PVDF membranes with a method similar to that ofScarnoutzos. The resulting filters did not wet with water or hexane. Thedisadvantage of the Moya filters is that air permeability of the treatedfilters was lower than the untreated substrates, and the fluorinatedmonomer is expensive.

[0011] Sugiyama et al. (U.S. Pat. No. 5,462,586) treated nylon fabricand PTFE membranes with solutions containing two different preformedfluoropolymers. The treated filters were resistant to water and oils.The durability of filters coated with preformed polymers is often lessthan that for filters where the coating is formed by polymerizing amonomer on the surface of the substrate, however.

[0012] Kenigsberg et al. (U.S. Pat. No. 5,156,780) treated a variety ofmembranes and fabrics with solutions of fluoroacrylate monomers andformed coatings on the substrate by polymerizing the monomer. Thecoating conferred oil and water repellency onto the substrate. However,the airflow through the treated membrane was reduced, compared to theuntreated membrane.

[0013] Hydrophobic media suitable for garments have been made byextruding mixtures of polypropylene or PTFE and the fluorochemicaloxazolidinone as disclosed in U.S. Pat. No. 5,260,360. The media made byextrusion tend to have relatively low air permeability.

[0014] In copending U.S. application Ser. No. 09/323,709 filed Jun. 1,1999 (incorporated herein by reference in its entirety), oleophobic andhydrophobic filters are prepared by forming a polydimethylsiloxanecoating on a polymeric substrate by polymerizing vinyl terminatedsiloxane with a crosslinker such as hydrosilicon in the presence of acatalyst.

[0015] In copending U.S. application Ser. No. 09/778,630 filed Feb. 7,2001 (incorporated herein by reference in its entirety), oleophobic andhydrophobic filters are prepared by forming a fluorosulfone oligomercoating on a substrate, such as a hydrophobic or hydrophilic membrane orother filtration medium.

[0016] In copending Australian Application Number PR5843 filed Jun. 20,2001 (incorporated herein by reference in its entirety), hollow fibermembranes for use in microfiltration are prepared from a terpolymer oftetrafluoroethylene, vinylidene fluoride, and hexafluoropropylene.

SUMMARY OF THE INVENTION

[0017] There is a need for a hydrophobic filter, which is inexpensiveand has high air permeability. Specifically, there is a need for afilter medium that is hydrophobic and that may be readily andreproducibly produced through simple casting solution phase inversionprocesses.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1a provides a SEM image of the skin side of the membrane ofExample 1.

[0019]FIG. 1b provides a SEM image of the dull side of the membrane ofExample 1.

[0020]FIG. 2a provides a SEM image of the skin side of the membrane ofExample 2.

[0021]FIG. 2b provides a SEM image of the dull side of the membrane ofExample 2.

[0022]FIG. 2c provides a SEM image of the cross-section of the membraneof Example 2.

[0023]FIG. 3a provides a SEM image of the skin side of the membrane ofExample 3.

[0024]FIG. 3b provides a SEM image of the dull side of the membrane ofExample 3.

[0025]FIG. 3c provides a SEM image of the cross-section of the membraneof Example 3.

[0026]FIG. 4a provides a SEM image of the skin side of the membrane ofExample 4.

[0027]FIG. 4b provides a SEM image of the dull side of the membrane ofExample 4.

[0028]FIG. 5a provides a SEM image of the skin side of the membrane ofExample 5.

[0029]FIG. 5b provides a SEM image of the dull side of the membrane ofExample 5.

[0030]FIG. 5c provides a SEM image of a cross-section of the membrane ofExample 5.

[0031]FIG. 6a provides a SEM image of the skin side of the membrane ofExample 6.

[0032]FIG. 6b provides a SEM image of the dull side of the membrane ofExample 6.

[0033]FIG. 6c provides a SEM image of a cross-section of the membrane ofExample 6.

[0034]FIG. 7a provides a SEM image of the skin side of the membrane ofExample 7.

[0035]FIG. 7b provides a SEM image of the dull side of the membrane ofExample 7.

[0036]FIG. 7c provides a SEM image of a cross-section of the membrane ofExample 7.

[0037]FIG. 8 provides a SEM image of a cross-section of the membrane ofExample 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0038] The following description and examples illustrate a preferredembodiment of the present invention in detail. Those of skill in the artwill recognize that there are numerous variations and modifications ofthis invention that are encompassed by its scope. Accordingly, thedescription of a preferred embodiment should not be deemed to limit thescope of the present invention.

[0039] The preferred embodiments provide hydrophobic filters that haveasymmetric or isotropic structures, high gas permeabilities, and thatrepel water. The preferred embodiments also include methods of preparingsuch filters by casting solution phase inversion.

[0040] The filter media are prepared using a fluorothermoplastic, suchas a terpolymer of tetrafluoroethylene (TFE), vinylidene fluoride (VDF),and hexafluoropropylene (HFP). The filters have high permeabilities forairflow and reject liquid water, as evidenced by high water penetrationpressures. The filters are useful, for example, as air filters or ventfilters for intravenous (IV) or other medical devices.

[0041] Introduction

[0042] Casting solution phase inversion is a process wherein a castingdope is spread in a film over a smooth surface, then the film, ornascent membrane, is passed through a quenching (or coagulating)solution, e.g., water, to extract the water miscible components from themembrane. In preferred embodiments, such techniques are used to preparemembranes from a solution or dispersion of PTFE, PVDF, and HFP.

[0043] The Polymeric Substrate

[0044] The membranes of preferred embodiments may be prepared from aterpolymer of tetrafluoroethylene, hexafluoropropylene, and vinylidenefluoride (hereinafter referred to as “fluorothermoplastic terpolymer”).Preferably, the terpolymer includes from about 20 or less to about 65wt. % or more vinylidene fluoride, more preferably from about 25, 30, or35 to about 40, 45, 50, 55, or 60 wt. % vinylidene fluoride, and mostpreferably about 36.5 wt. % vinylidene fluoride. Preferably, theterpolymer includes from about 30 or less to about 70 wt. % or moretetrafluoroethylene, more preferably from about 35 or 40 to about 45,50, 55, 60, or 65 wt. % tetrafluoroethylene, and most preferably about44.6 wt. % tetrafluoroethylene. Preferably, the terpolymer includes fromabout 10 or less to about 20 wt. % or more hexafluoroethylene, morepreferably from about 11, 12, 13, 14, 15, 16, 17, or 18 to about 19 wt.% hexafluoroethylene, and most preferably about 18.9 wt. %hexafluoroethylene.

[0045] Suitable fluorothermoplastic terpolymers include Dyneon™Fluorothermoplastics available from Dyneon LLC of Oakdale Minn.Particularly preferred is Dyneon™ THV 220A, which is composed oftetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride in theform of an agglomerate that is hydrophobic and is soluble in ketones,esters, and ethers. Dyneon™ THV 220A has a melting point of 120° C., amelt flow index of 20 (265° C./5 kg), a specific gravity of 1.95 g/cc,tensile at break of 20 MPa, elongation at break of 600%, and flexuralmodulus of 80 MPa. Other Dyneon™ fluorothermoplastic terpolymers includeDyneon™ THV 220, Dyneon™ THV 410, Dyneon™ THV 500, and Dyneon™ THV X610. The Dyneon™ fluorothermoplastic terpolymers with lower numbers,e.g., THV 220, are generally preferred due to the ease with whichsatisfactory casting dopes may be prepared. The Dyneon™fluorothermoplastic terpolymers with higher numbers, e.g., THV 410, THV500, and THV X 610, have progressively higher percentages oftetrafluoroethylene. The higher the percentage of tetrafluoroethylene,the less soluble the fluorothermoplastic terpolymer is in typicalsolvents, thus making selection of an appropriate solvent or solventsand preparation of casting solutions or dispersions more difficult. Forexample, satisfactory solutions of Dyneon™ THV 410 may be prepared usingn-propylamine or di-n-propylamine.

[0046] A single fluorothermoplastic terpolymer or combination of afluorothermoplastic terpolymer and one or more additional polymers maybe preferred. The additional polymer may include anotherfluorothermoplastic terpolymer, or any other suitable polymer. Suitablepolymers may include any suitable homopolymer, copolymer, or terpolymer,including but not limited to polysulfone, polyethersulfone (PES),polyarylsulfone, fluorinated polymers such as polyvinylidene fluoride(PVDF), polyolefins including polyethylene and polypropylene,polytetrafluoroethylene (PTFE or Teflon™),poly(tetrafluoroethylene-co-ethylene) (ECTFE or Halar™), acryliccopolymers, polyamides or nylons, polyesters, polyurethanes,polycarbonates, polystyrenes, polyethylene-polyvinyl chloride,polyacrylonitrile, cellulose, and mixtures or combinations thereof.

[0047] The fluorothermoplastic terpolymer may be subjected to apretreatment, for example grafting or crosslinking, prior to forming amembrane casting dope, or may be subjected to a post-treatment, forexample grafting or crosslinking, after a membrane is cast. There is noparticular molecular weight range limitation for suitablefluorothermoplastic terpolymer. Likewise, there is no particularlimitation on the weight ratio of the tetrafluoroethylene,hexafluoropropylene, and vinylidene fluoride monomers in thefluorothermoplastic terpolymer. Various molecular weights and/ordifferent monomer ratios may be preferred for membranes to be used forcertain applications.

[0048] The Membrane

[0049] Membranes that may be prepared in accordance with preferredembodiments include ultrafiltration and microfiltration asymmetric andisotropic membranes. The term “isotropic” as used herein relates to amembrane possessing a constant pore size across the thickness of themembrane. The term “asymmetric” as used herein relates to a membranepossessing a pore size gradient. That is, asymmetric membranes possesstheir smallest or finest pores in or adjacent to one surface of themembrane, generally referred to as the “skin” surface or “shiny” side ofthe membrane. The increase in pore size between the skin surface and theopposite surface of the membrane is generally gradual, with the smallestpore size nearest the skin surface and the largest pores being found ator adjacent to the opposite, coarse-pored surface, generally referred toas the “open” surface or the “dull” side of the membrane. Anothervariety of asymmetric membrane, commonly described as having a“funnel-with-a-neck” structure, includes both an asymmetric region andan isotropic region, the isotropic region having a uniform pore size.The isotropic region typically extends from the skin surface of themembrane through about 5-80% of the thickness of the membrane, morepreferably from about 15-50% of the thickness of the membrane. Symmetricmembranes exhibit a substantially uniform pore size throughout thethickness of the membrane. Although asymmetric membranes are generallypreferred for filtering applications, in certain embodiments a symmetricmembrane may be preferred.

[0050] Some filtration membranes have a layer of relatively small poreson one side (termed herein a “skin”) when compared to the other side,while other membranes do not contain this type of layer (termed herein“skinless”). A skinned membrane is typically created by quenching apolymeric casting dope of sufficient polymer concentration in a strongnonsolvent. The resultant membrane has considerably smaller pores on the“skin” face than on the opposite face.

[0051] The membranes of preferred embodiments have a porous supportingstructure between the two sides of the membrane. The nature of theporous supporting structure of a membrane may depend on the compositionof the casting dope and the quenching bath. The supporting structure mayinclude closed cells, open cells of substantially the same pore sizefrom one side of the membrane to the other, open cells with a gradationof pore sizes from one side of the membrane to the other, or finger-typestructures, generally referred to as “macrovoids.” Macrovoids typicallywill vary substantially in size from the surrounding porosity, andgenerally do not communicate with surface pores. In a preferredembodiment, the porous supporting structure includes a network ofstructural surfaces capable of contacting a filter stream, definedherein as including any fluid substance, including liquids and gases,that passes through the membrane via the porous supporting structure. Inpreferred embodiments, the supporting structure includes reticulatednetwork of flow channels. In particularly preferred embodiments, thesupporting structure includes either no macrovoids or an insignificantnumber of macrovoids.

[0052] Whether the membrane has an asymmetric or funnel-with-a-neckstructure may depend upon several factors involved in the preparation ofthe membrane, including the type and concentration of the polymer, thesolvent, and the nonsolvent; the casting conditions such as the knifegap, and the dope temperature; environmental factors such as theexposure time between casting and quenching, and the humidity of theexposure atmosphere; and the composition and temperature of the quenchbath.

[0053] In particularly preferred embodiments, the membranes have anasymmetric structure wherein an increase in pore size is observed fromone side of the membrane to the other. In various embodiments, theasymmetry in pore size between the skin side and dull side of themembrane may range from about 1:1.5 or less, 1:2, 1:3, 1:4, 1:5, 1:10,1:20, 1:50, 1:100, 1:500, 1:1000 or greater.

[0054] Suitable membranes may typically possess porositiescharacteristic of ultrafiltration or microfiltration membranes.Membranes within the ultrafiltration range preferably possess molecularweight cutoffs of from about 10,000 Daltons to about 1,000,000 Daltonsand may have pore diameters from about 0.001 μm to about 0.05 μm on theskin side of the membrane. Microfiltration membranes typically possesspore diameters of at least about 0.01 or about 0.05 μm to about 5, 6, 7,8, 9, 10, 15, or 20 μm on the skin side of the membrane. While themembranes of preferred embodiments typically possess porositiescharacteristic of ultrafiltration or microfiltration membranes, incertain embodiments porosities less than 0.001 μm or greater than 20 μm.

[0055] The fluorothermoplastic terpolymer membranes that may be preparedaccording to the preferred embodiments may be in any suitable shape orform, including, but not limited to, sheet and hollow fiber cast polymermembranes. Suitable membranes further include both those membranes thatare cast from a single polymer solution or dope, referred to as“integral” membranes, as well as non-integral or composite membranesthat are cast from more than one polymer solution or dope to form alayered or composite membrane. Composite membranes may also be assembledfrom two or more fully formed membranes after casting, for example, bylamination or other suitable methods. Preparation of composite membranesby lamination is discussed further in copending U.S. patent applicationSer. No. 09/694,120 filed on Oct. 20, 2000 and entitled “LAMINATES OFASYMMETRIC MEMBRANES,” and Ser. No. 09/694,106 also filed on Oct. 20,2000 and also entitled “LAMINATES OF ASYMMETRIC MEMBRANES,” each ofwhich is incorporated herein by reference in its entirety.

[0056] The filtration media prepared from the membranes of preferredembodiments may include composites, such as, for example, compositeshaving different layers of various media, composites having multiplelayers of the same medium, or composites having layers of the samemedium, but of different pore sizes, porosities, geometries,orientations, and the like. Suitable media that may be used incombination with the fluorothermoplastic terpolymer membranes ofpreferred embodiments include, but are not limited to, hollow fibermedia, melt blown or other nonwoven media, woven media, extruded media,and sedimented media. Suitable melt blown media include, but are notlimited to, media including polymers such as polyester, polypropylene orECTFE, polyethylene or other polyolefins, nylon, and the like, and arecommercially available from U.S. Filter/Filterite Division, Timonium,Md.

[0057] The Casting Dope

[0058] The fluorothermoplastic terpolymer membranes of the preferredembodiments are preferably prepared from stable, clear homogeneoussolutions and/or stable colloidal dispersions. The solutions ordispersions can be prepared through the use of solvents alone, or incombination with nonsolvents.

[0059] The membranes are generally prepared from a casting solution ordispersion (also referred to as a casting dope) of thefluorothermoplastic terpolymer, along with particular concentrations ofpolymer solvents and nonsolvents. The concentration of the polymer inthe casting dope is low enough to form a substantially all-reticulatedstructure, but high enough to produce a coherent membrane. If thepolymer concentration is too low, the resulting membrane can haveinadequate coherency and, in the extreme case, only dust is formed. Ifthe polymer concentration is too high, the membrane structure is notsubstantially reticulated and can contain at least some granularstructures.

[0060] Although the appropriate concentration of the fluorothermoplasticterpolymer varies somewhat depending upon the particular conditionsused, (e.g., solvents, etc.), the fluorothermoplastic terpolymerconcentration in the casting dope is generally from about 5 wt. % orless to about 30 wt. % or more. Typically, the casting dope containsfrom about 10 wt. % to about 25 wt. % fluorothermoplastic terpolymer,preferably the casting dope includes about 11, 12, 13, 14, or 15 wt. %to about 17, 18, 19, or 20 wt. % fluorothermoplastic terpolymer, andmost preferably the polymer is present in the casting dope at about 16wt. %.

[0061] In certain embodiments, it may be desirable for the casting dopeto contain additional substances other than the terpolymer, the solvent,and the nonsolvent.

[0062] It has been found that a stable, clear homogeneous castingsolution or stable colloidal dispersion can be obtained by dissolvingthe polymer in a suitable solvents, such as ketones, esters, and ethers.Preferred solvents include, for example, dimethylformamide (DMF),n-butanol, and acetone. Any suitable solvent may be used, however.Examples of other suitable solvents include dipolar aprotic solventssuch as methylene chloride, N-methylpyrrolidone, dimethylformamide,dimethylacetamide, dioxane, dimethylsulfoxide, chloroform,tetramethylurea, or tetrachloroethane, and their mixtures. It isgenerally not preferred to use a highly volatile solvent as the solesolvent in preparing the casting dope, because such solvents may yielddense films with unsatisfactory porosity. However, in certainembodiments it may be desirable to use a highly volatile solvent. Highlyvolatile solvents may also be preferred components of cosolvent systemswhen present with a less volatile solvent, as discussed below.

[0063] The solvent that may be employed to prepare preferred membranesis typically present at about 15 wt. % or less or about 95 wt. % or moreof the casting dope; generally from about 20 wt. % to about 90 wt. %;preferably from about 30, 40, 50, 60, or 70 wt. % to about 86, 87, 88,or 89 wt. %, more preferably from about 71, 72, 73, or 74 wt. % to about81, 82, 83, 84, or 85 wt. %; and most preferably from about 75, 76, or77 wt. % to about 78, 79, or 80 wt. % of the solution. The preciseamount of solvent to be used is determined by the particular castingdope, including the particular polymer, nonsolvent, and the otherconditions of the method of preparation of the particular membrane ofinterest.

[0064] In preferred embodiments, a cosolvent system is used. Suchcosolvent systems generally include two or more components of differentvolatilities. By selecting solvents of appropriate volatility andcontrolling the proportion of each solvent in the cosolvent system, thepore size and degree of asymmetry of the resulting membrane may betightly controlled. Solvents of higher volatility generally yieldmembranes having a greater degree of asymmetry than do solvents of lowervolatility. Solvents of higher volatility also tend to yield membraneshaving a smaller minimum pore size than solvents of lower volatility. Byincreasing the percentage of the more volatile solvent or solvents inthe cosolvent system, the degree of asymmetry and pore size may bereduced. When a cosolvent system is used, it is generally preferred thatone or more higher volatility components are present in combination withone or more lower volatility components. The terms “higher volatility”and “lower volatility” refer to relative volatilities when comparing twoor more components. Higher volatility components generally have a vaporpressure greater than about 4 mm/g@20° C. Lower volatility componentsgenerally have a vapor pressure less than about 4 mmHg@20° C. In certainembodiments, however, it may be preferred to use a component having avapor pressure of greater than about 4 mmHg@20° C. as the “lowervolatility component.” Likewise, in certain embodiments it may bepreferred to use a component having a vapor pressure of less than about4 mmHg@20° C. as the “higher volatility component.”

[0065] A nonsolvent may be added to the casting dope. In preferredembodiments, the nonsolvent includes, but is not limited to, water,alcohols, for example, methanol, ethanol, isopropanol, 2-methoxyethanol,amyl alcohols such as t-amyl alcohol, hexanols, heptanols, and octanols;alkanes such as hexane, propane, nitropropane, heptane, and octane; andketones, carboxylic acids, ethers and esters such as propionic acid,butyl ether, ethyl acetate, and amyl acetate, di(ethyleneglycol)diethylether, di(ethyleneglycol) dibutylether, polyethylene glycol,methylethyl-ketone, methylisobutylketone, glycerol, diethyleneglycol,and their mixtures. By adjusting the proportion of nonsolvent in thecasting dope, the degree of asymmetry and pore size may be controlled.While it is generally preferred to use a cosolvent system as describedabove to control the degree of asymmetry and pore size because of thegreater degree of control that may generally be obtained, in certainembodiments it may be preferred to use a solvent/nonsolvent systeminstead.

[0066] The components of the casting dope may be combined in anysuitable order. However, it is generally convenient to add nonsolvent,if it is to be used, to the casting dope at the same time as thefluorothernoplastic terpolymer is dissolved in the solvent.

[0067] The total amount of nonsolvent which may be employed to preparethe membrane may vary for different nonsolvents, however the preferredamount of nonsolvent is typically from about 1 wt. % or less to about 50wt. % or more of the casting dope; preferably from about 2 wt. % toabout 20, 20, or 40 wt. %; more preferably from about 3 wt. % to about15 wt. %; and most preferably from about 4, 5, 6, 7, 8, or 9 wt. % toabout 10, 11, 12, 13, or 14 wt. % of the casting dope. Selection of theprecise amount of nonsolvent to be used is based on the particularcasting dope, including the polymer, solvent, and the other conditionsof the method of preparation of the particular membrane of interest.

[0068] The Casting Process

[0069] In general, the overall method of preparing preferredfluorothermoplastic terpolymer membranes includes the steps of providinga casting dope comprising a solution or stable colloidal dispersion. Inpreferred embodiments, the casting dope is cast as a thin film andexposed to a gaseous environment. Once the casting dope has been exposedto the gaseous environment, it is quenched in a quench bath. Afterquenching, the resulting fluorothermoplastic terpolymer membrane may berinsed in a suitable solvent, then air- or oven-dried.

[0070] The fluorothermoplastic terpolymer membranes of preferredembodiments can be cast using any conventional procedure wherein thecasting solution or dispersion is spread in a layer onto a porous ornonporous support from which the membrane later can be separated afterquenching, or upon which the membrane may be retained. The membranes canbe cast manually by being poured, cast, or spread by hand onto a castingsurface followed by application of a quench liquid onto the castingsurface. Alternatively, the membranes may be cast automatically bypouring or otherwise casting the solution or dispersion onto a movingbelt. The casting solution or dispersion may be any suitabletemperature, i.e., room temperature, or any temperature at which thecasting dope is capable of being cast. Preferably, the temperature isbetween about 10° C. and about 38° C., more preferably between about 16°C. and about 32° C., and most preferably between about 21° C. and about26° C. In preferred embodiments, the temperature is preferably aboutroom temperature.

[0071] One type of moving belt support is polyethylene-coated paper. Incasting, particularly in automatic casting, mechanical spreaders can beused. Mechanical spreaders include spreading knives, a doctor blade orspray/pressurized systems. A preferred spreading device is an extrusiondie or slot coater which has a chamber into which the castingformulation can be introduced. The casting dope is then forced out ofthe chamber under pressure through a narrow slot. Membranes may also becast by means of a doctor blade with a knife gap typically from about1000 μm or more down to about 900, 800, 700, 600, 500, 400, 350 μm orless. Preferably, the knife gap is about 300, 250, 200, 150, 100 μm orless, and most preferably it is 75 μm or less. The relationship betweenthe knife gap at casting and the final thickness of the membrane is afunction of the composition and temperature of the casting dope, theduration of exposure to the gaseous environment, such as humid air, therelative humidity of the air during exposure. In addition, thetemperature of the quench bath and many other factors can affect theoverall thickness of the final membrane. Membranes typically shrink upongelling, losing from about 20% to about 80% of their thickness.

[0072] In preferred embodiments, the cast film is exposed to a gaseousenvironment, such as air, sufficiently long to induce formation ofsurface pores. Another factor that is important to the manufacture ofthe membranes of the preferred embodiments is the exposure time andexposure conditions that exist between casting and quenching the castingdope. In certain embodiments, the casting solution or dispersion may beexposed to humid air after casting but before quenching. Ambienthumidity is acceptable as are other humidity conditions. In a preferredembodiment, the gaseous environment has a relative humidity of betweenabout 50% and about 75%, preferably between about 55% and about 70%,more preferably between about 60% and about 65%, and most preferablyabout 60%. In certain embodiments, the air may be circulated to enhancecontact with the cast solution or dispersion. The gaseous atmosphere maybe any suitable temperature, but is typically between about 10° C. andabout 30° C., preferably between about 15° C. and about 25° C., and morepreferably between about 20° C. and about 25° C. Most preferably, thetemperature is from about room temperature to slightly higher than roomtemperature.

[0073] The method of preparing the membranes of the preferredembodiments typically involves a period of exposure to the gaseousenvironment after casting and before quenching. The exposure time to thegaseous environment is preferably between about 0 seconds and about 10seconds or more. More preferably, the exposure time is between about 1second and about 5 seconds, and most preferably between about 1 secondand about 2 seconds. Increasing the air exposure time over this rangetends to increase permeability and pore size of the resulting membrane.

[0074] Following casting and exposure to a gaseous environment, such asair, the cast dispersion or solution is quenched or coagulated. In apreferred embodiment, quenching is accomplished by transporting the castmembrane on a moving belt into the quenching liquid, such as a waterbath or a mixture of methanol and water. Most commonly, the quenching orcoagulating liquid is water, however, any suitable liquid or mixture ofliquids that is not a solvent for the resulting fluorothermoplasticterpolymer membrane may be used. In the quench or coagulating bath, thepolymer precipitates or coagulates to produce the desired porousreticulated structure.

[0075] The temperature of the quench bath can affect the porosity of themembrane. In general, warmer quench baths result in more porousmembranes. Generally, a wide temperature range may be utilized in thequenching step, ranging from about −2° C. to about 40° C., preferablyfrom about 5° C. to about 30° C., and more preferably from about 10° C.to about 25° C. The lower temperature limit is determined by thefreezing point of the particular quench liquid. Preferably, the quenchliquid is water and the quenching temperature is about 20° C. Thetemperature of the quench bath may cause marked changes in the porediameters of the membrane. Where higher quench temperatures areutilized, the membranes possess larger pores. Conversely, where lowertemperatures are utilized, smaller pores form.

[0076] Membranes are recovered from the quench bath in the conventionalmanner by physical removal. The resulting fluorothermoplastic terpolymermembrane is typically washed free of solvent and may be dried to expeladditional increments of solvent, diluent, and quench liquid. Washingliquids include any suitable liquid that is not a solvent for theresulting fluorothermoplastic terpolymer membrane. In a preferredembodiment, the rinse liquid is deionized water. The membranes may bedried by air-drying or oven drying. In a preferred embodiment, thefluorothermoplastic terpolymer membrane is air dried at roomtemperature. If drying at elevated temperature, e.g., in an oven, isperformed, the temperature is typically selected such that exposure ofthe membrane to that temperature does not substantially affect theperformance characteristics of the membrane, for example, by melting thepolymer comprising the membrane. Drying temperatures from below roomtemperature to slightly below the melting point of thefluorothermoplastic terpolymer are typically used. Preferably, dryingtemperatures ranging from about 50° C. to about 100° C., more preferablyfrom about 60° C. to about 90° C., and most preferably from about 70° C.to about 80° C. are used. It is preferred to circulate the air in ovenso as to ensure rapid and even drying. The humidity of the air in theoven need not be controlled. However, drying tends to be more rapid atlower humidity levels.

[0077] The fluorothermoplastic terpolymer membranes produced by themethods described above may be from about 100 μm thick or less to about500 μm thick or more. Preferably, the thickness of the membrane is about500 μm, including any supporting material, and is preferably about 75 μmnot including any supporting material. However, any useful thickness ofmembrane can be prepared by varying the process parameters following theteachings herein.

[0078] Membrane Architecture

[0079] Fluorothermoplastic terpolymer membranes of the preferredembodiments are typically made from tetrafluoroethylene,hexafluoropropylene, and vinylidene fluoride terpolymer. The membranesmay be isotropic or asymmetric. Asymmetries in pore size typically rangefrom about 1:1.5 or less to about 1:20 or more, more preferably fromabout 1:1.5 to about 1:10, and most preferably from about 1:2 to about1:5. Pore sizes preferably range from about 0.001 μm or less to about 20μm or more, more preferably from about 0.005 μm to about 10 μm, and mostpreferably from about 0.005 μm to about 5 μm.

[0080] Pore diameter in preferred fluorothermoplastic terpolymermembranes is generally estimated by porometry analysis. Porometrymeasurements give the “mean flow pore diameter” (MFP diameter, alsoreferred to as MFP size) of the membrane. The MFP diameter is theaverage size of the limiting pores in a membrane. The MFP diameter isbased on the pressure at which air flow begins through a pre-wettedmembrane (the bubble point pressure) compared to the pressure at whichthe air flow rate through a pre-wetted membrane is half the air flowrate through the same membrane when dry (the mean flow pore pressure). ACoulter Porometer, manufactured by Beckman Coulter Inc. of Fullerton,Calif., is typically used for analysis of MFP diameter and minimum poresize. The membranes of the preferred embodiments typically have MFPdiameters ranging from about 0.01 μm or less to about 5 μm or more,preferably from about 0.05 μm to about 4 μm, more preferably from about0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, μm to about3.5 μm, and most preferably from about 0.20, 0.25, 0.30, 0.35, 0.40,0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, or 0.95 μmto about 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.6,2.8, 3.0, 3.2, or 3.4 μm. However, in certain embodiments higher orlower MFP diameters may be preferred.

[0081] Hydrophobic filters resist being penetrated by water. Suchpenetration can, however, be induced by imposing a pressure sufficientto penetrate water into the filter pores. The magnitude of the pressurerequired to penetrate the water varies indirectly with the sizes of thepores. Measurement of the penetration pressure therefore provides agauge of the largest size pores present in the filter.

[0082] Composites Including Fluorothermoplastic Terpolymer Membranes

[0083] In preferred embodiments, the fluorothermoplastic terpolymermembranes are fabricated into composite membranes or filters. Suchcomposites have multiple layers and are useful in a variety ofseparation applications. In many cases, the various layers of acomposite membrane or filter each impart different desirable properties.For example, in some applications, an extremely thin membrane may haveadvantageous flow rates in separations of very small particles, gasses,and the like. Yet such a thin membrane may be fragile and difficult tohandle or to package into cartridges. In such cases, the fragile, thinlayer membrane may be combined with a support material as a backing toform a composite having improved strength and handling characteristicswithout sacrificing the separations properties of the thin layermembrane. Other desirable properties imparted by forming a compositemembrane may include increased burst strength, increased tensilestrength, increased thickness, and superior prefiltration capability.

[0084] Composite membranes or filters incorporating the membranes of thepreferred embodiments may be prepared using lamination techniques. In atypical lamination process, for example, the membrane and one or moreadditional sheets are layered together to form a stack, which is thenlaminated into an integral composite under application of heat andpressure. An adhesive substance may be placed in between the membraneand the adjacent sheet prior to lamination to facilitate binding andlamination of the membrane and sheet to each other.

[0085] Another approach to preparing composite membranes is to cast orform one membrane layer in situ on top of another layer such as, forexample, a woven or nonwoven support. Suitable supports includepolymeric nonwoven materials. In a preferred embodiment, the support isa nonwoven polyester material. However, any suitable substrate may beused. Alternatively, the membrane may be cast or formed on top ofanother layer, such as, for example, a membrane or other backingmaterial.

[0086] Any fluorothermoplastic terpolymer membrane that may be preparedaccording to the preferred embodiments by a casting or other process,that possesses the pore size criteria described above is generallysuitable for use in the present invention.

EXAMPLES

[0087] The following examples are provided to illustrate the preferredembodiments. However, such examples are merely illustrative and are notintended to limit the subject matter of the application.

Example 1

[0088] A casting solution was prepared containing 15 wt. % Dyneon™ THV220A, 5 wt. % n-butanol (Crown Chemical of San Diego, Calif.) and 80 wt.% DMF (E. I. du Pont de Nemours and Company of Wilmington, Del.) as thesolvents. The Dyneon™ THV 220A was added to the solvents and the mixturewas heated to about 50 to 60° C. with mixing. A membrane was cast onto amoving belt of polyethylene coated paper using a casting knife with aknife gap of approximately 0.20 mm (approximately 8 mils). The solutionwas cast at room temperature. The membrane was exposed to air (at roomtemperature, 60% R.H.) for approximately 1 to 2 seconds beforequenching. The membrane was quenched in a water bath having atemperature of about 20° C. for more than 10 minutes.

[0089] After quenching, the membrane was rinsed with deionized water andthen air-dried at room temperature for approximately 30 minutes. Themembrane was hydrophobic (not wettable by water). The membrane wastested for water penetration pressure and water flow at 69 kPa (10 psid)on a 90-mm diameter disk. The membrane exhibited an 1100 ml/min waterflow rate and a water penetration of 18 psi. The surfaces of themembranes were examined by Scanning Electron Microscopy (SEM). FIG. 1aprovides a SEM image of the skin side of the membrane and FIG. 1bprovides a SEM image of the dull side of the membrane. The membrane washighly asymmetric (degree of asymmetry of approximately 50:1) and had amean flow pore size of 1.02 μm.

Example 2

[0090] A casting solution was prepared containing 15 wt. % Dyneon™THV220A and 85 wt. % DMF as the solvent using the method as described inExample 1. The resulting membrane was hydrophobic (not wettable bywater). The membrane was tested for water penetration pressure and waterflow as described in Example 1. The membrane exhibited a 7000 ml/minwater flow rate and a water penetration of 6 psi. FIG. 2a provides a SEMimage of the skin side of the membrane, FIG. 2b provides a SEM image ofthe dull side of the membrane, and FIG. 2c provides a SEM image of thecross-section of the membrane. The membrane was asymmetric (degree ofasymmetry of approximately 50:1) and had a mean flow pore size of 3.3μm.

Example 3

[0091] A casting solution was prepared containing 15 wt. % Dyneon™THV220A, 10% n-butanol, and 75 wt. % DMF as the solvent using the methodas described in Example 1. The resulting membrane was hydrophobic (notwettable by water). The membrane was tested for water penetrationpressure as described in Example 1. The membrane exhibited a waterpenetration of 50 psi. FIG. 3a provides a SEM image of the skin side ofthe membrane, FIG. 3b provides a SEM image of the dull side of themembrane, and FIG. 3c provides a SEM image of the cross-section of themembrane. The membrane was asymmetric (degree of asymmetry ofapproximately 5:1) and had a mean flow pore size of 0.18 μm.

Example 4

[0092] A casting solution was prepared containing 14 wt. % Dyneon™ THV220A, 3% n-butanol, and 83 wt. % DMF as the solvent using the method asdescribed in Example 1. The resulting membrane was hydrophobic (notwettable by water). The membrane was tested for water penetrationpressure and water flow as described in Example 1. The membraneexhibited a 1200 ml/min water flow rate and a water penetration of 15psi. FIG. 4a provides a SEM image of the skin side of the membrane andFIG. 4b provides a SEM image of the dull side of the membrane. Themembrane was asymmetric (degree of asymmetry of approximately 20:1) andhad a mean flow pore size of 0.1 μm.

Example 5

[0093] A casting solution was prepared containing 16 wt. % Dyneon™THV220A, 8% n-butanol, and 76 wt. % DMF as the solvent using the methodas described in Example 1. The resulting membrane was hydrophobic (notwettable by water). The membrane was tested for water penetrationpressure and water flow as described in Example 1. The membraneexhibited a 1500 ml/min water flow rate and a water penetration of 32psi. FIG. 5a provides a SEM image of the skin side of the membrane, FIG.5b provides a SEM image of the dull side of the membrane, and FIG. 5cprovides a SEM image of a cross-section of the membrane. The membranewas asymmetric (degree of asymmetry of approximately 10:1) and had amean flow pore size of 0.76 μm.

[0094] A comparison of MFP size, water flow, and water penetration forthe membranes of Examples 1-5 is provided in Table 1. The data indicatethat as the percentage of the more volatile component (n-butanol) in thecasting dope is increased, MFP size decreases along with water flowrate, and that the water penetration pressure increases. TABLE 1 Waterflow % MFP (ml/min for 90 mm Water Penetration Example # Butanol Size(μm) disc @ 10 psi) Pressure (psi) 2 0 3.3 7,000 6 4 3 1.1 1,200 15 1 51.02 1,100 18 5 8 0.76 1,500 32 3 10 0.18 NA 50

Example 6

[0095] A casting solution was prepared containing 16 wt. % Dyneon™THV220A, 5% acetone (Sigma-Aldrich Company of St. Louis, Mo.), and 81wt. % DMF as the solvent using the method as described in Example 1. Theresulting membrane was hydrophobic (not wettable by water). The membranewas tested for water penetration pressure and water flow as described inExample 1. The membrane exhibited a 1440 ml/min water flow rate and awater penetration of 20 psi. FIG. 6a provides a SEM image of the skinside of the membrane, FIG. 6b provides a SEM image of the dull side ofthe membrane, and FIG. 6c provides a SEM image of a cross-section of themembrane. The membrane was asymmetric (degree of asymmetry ofapproximately 20:1) and had a mean flow pore size of 0.475 μm.

Example 7

[0096] A casting solution was prepared containing 14 wt. % Dyneon™THV220A, 10% acetone, and 75 wt. % DMF as the solvent using the methodas described in Example 1. The resulting membrane was hydrophobic (notwettable by water). The membrane was tested for water penetrationpressure and water flow as described in Example 1. The membraneexhibited a 960 ml/min water flow rate and a water penetration of 40psi. FIG. 7a provides a SEM image of the skin side of the membrane, FIG.7b provides a SEM image of the dull side of the membrane, and FIG. 7cprovides a SEM image of a cross-section of the membrane. The membranewas asymmetric (degree of asymmetry of approximately 20:1) and had amean flow pore size of 0.356 μm.

[0097] A comparison of MFP size, water flow, and water penetration forthe membranes of Examples 2, 6, and 7 is provided in Table 1. The dataindicate that as the percentage of the more volatile solvent (acetone)in the casting dope is increased, MFP size decreases along with waterflow rate and the water penetration pressure increases. Wate flow(ml/min for 90 mm) Water Penetration Example # % acetone MFP Size (μm)disc @ 10 psi Pressure (psi) 2 0 3.3 7,000 6 6 5 0.47 1,440 20 7 10 0.36  960 40

Example 8

[0098] A casting solution was prepared containing 16 wt. % Dyneon™ THV220A, 8% n-butanol, and 76 wt. % DMF as the solvent as in Example 5. Amembrane was cast onto a polyester nonwoven support (Reemay R-125-Favailable from BBA Nonwovens Reemay, Inc. of Old Hickory, Tenn.) using acasting knife with a knife gap of approximately 0.20 mm (8 mil). Castingconditions were the same as in Example 5. The membrane was hydrophobic(not wettable by water). The membrane was tested for MFP size, waterpenetration pressure, and water flow as in Example 5, and gave similarresults as the unsupported membrane in Example 5. FIG. 8 provides a SEMimage of a cross-section of the membrane.

[0099] The preferred embodiments have been described in connection withspecific embodiments thereof. It will be understood that it is capableof further modification, and this application is intended to cover anyvariations, uses, or adaptations of the invention following, in general,the principles of the invention and including such departures from thepresent disclosure as come within known or customary practices in theart to which the invention pertains and as may be applied to theessential features hereinbefore set forth, and as fall within the scopeof the invention and any equivalents thereof.

What is claimed is:
 1. A flat sheet membrane, the membrane cast from adope comprising a solvent and a terpolymer of tetrafluoroethylene,vinylidene fluoride, and hexafluoropropylene, the membrane having afirst porous face having a first average pore diameter, a second porousface having a second average pore diameter, and a porous supportingstructure therebetween wherein the supporting structure comprises areticulated network of flow channels, and wherein the porous faces andthe porous supporting structure comprise a network of structuralsurfaces capable of contacting a filter stream.
 2. The membrane of claim1, wherein the membrane comprises an isotropic membrane.
 3. The membraneof claim 1, wherein the first and second average pore diameters have anasymmetry of at least about 2:1.
 4. The membrane of claim 1, wherein thefirst and second average pore diameters have an asymmetry of at leastabout 5:1.
 5. The membrane of claim 1, wherein the first and secondaverage pore diameters have an asymmetry of at least about 10:1.
 6. Themembrane of claim 1, wherein the first and second average pore diametershave an asymmetry of at least about 20:1.
 7. The membrane of claim 1,wherein the dope comprises a dispersion of the terpolymer in thesolvent.
 8. The membrane of claim 1, wherein the dope comprises ahomogeneous solution of the terpolymer in the solvent.
 9. A method forpreparing a membrane, the method comprising: providing a casting dopecomprising a solvent and a terpolymer of tetrafluoroethylene, vinylidenefluoride, and hexafluoropropylene; casting the dope to form a thin film;exposing the film to a humid atmosphere for a period of time sufficientto allow formation of surface pores; quenching the film in a quenchingbath; and recovering from the quenching bath a flat sheet membrane, themembrane comprising a terpolymer of tetrafluoroethylene, vinylidenefluoride, and hexafluoropropylene, the membrane having a first porousface having a first average pore diameter, a second porous face having asecond average pore diameter, and a porous supporting structuretherebetween wherein the supporting structure comprises a reticulatednetwork of flow channels, and wherein the porous faces and the poroussupporting structure comprise a network of structural surfaces capableof contacting a filter stream.
 10. The method of claim 9, furthercomprising: rinsing the membrane in a rinsing bath, wherein the rinsingstep is conducted after the quenching step.
 11. The method of claim 10,further comprising: drying the membrane at an elevated temperature. 12.The method of claim 10, further comprising: drying the membrane at roomtemperature.
 13. The method of claim 9, wherein the dope comprises ahomogeneous solution.
 14. The method of claim 9, wherein the dopecomprises a dispersion.
 15. The method of claim 9, wherein the dopefurther comprises a nonsolvent.
 16. The method of claim 15, wherein thenonsolvent comprises water.
 17. The method of claim 15, wherein the dopecomprises from about 1 wt. % to about 50 wt. % of nonsolvent.
 18. Themethod of claim 9, wherein the solvent is selected from the groupconsisting of ketones, esters, ethers, and mixtures thereof.
 19. Themethod of claim 9, wherein the solvent comprises n-butanol.
 20. Themethod of claim 9, wherein the solvent comprises acetone.
 21. The methodof claim 9, wherein the solvent comprises dimethylformamide.
 22. Themethod of claim 9, wherein the solvent comprises a first solvent havinga first vapor pressure and a second solvent having a second vaporpressure, and wherein the first vapor pressure and the second vaporpressure are different.
 23. The method of claim 9, wherein the dopecomprises from about 15 wt. % to about 95 wt. % of solvent.
 24. Themethod of claim 9, wherein the dope comprises from about 5 wt. % toabout 30 wt. % of the terpolymer.
 25. The method of claim 9, wherein thequenching bath comprises water.
 26. The method of claim 10, wherein therinsing bath comprises water.